In Shakespeare's Julius Caesar, Cassius is one of the conspirators that murdered Caesar. As lines are firmly drawn before battle in Act Five, scene one—between Antony and his army, and Cassius and Brutus and their army—Cassius tells Messala about his part in the war fighting under Pompey. In reference to Pompey:

Cassius...was the most famous of [his] clan...Cassius supported Pompey in the Battle of Pharsalus (48 B.C.), but Caesar forgave him and in 44 B.C. made him a praetor. Cassius was an active conspirator and one of the actual assassins of Julius Caesar in 44 B.C.

Cassius supported Pompey against Julius Caesar in a long war. Caesar defeated the leader of the opposition, Pompey, at Pharsalus, and it was in this war that Cassius supported not Caesar, but the other side. (Ironically, Caesar forgave Cassius his part in the war, but one day Cassius would lead the conspiracy to murder Caesar.) It would seem that Pompey was forced to fight Caesar when he would have preferred not to. In this case, Cassius is in the same situation.

Be thou my witness that, against my will, (80)

As Pompey was, am I compell'd to set

Upon one battle all our liberties.

As he faces yet another battle, Cassius' mention of Pompey to Messala recalls for him the past when Pompey put everything (all of his "liberties") "on the line" with the hopes of winning the battle. Cassius' indicates that it was not what Pompey wanted then; once again, however, the same situation has come before Cassius now, against his will—not fighting Caesar this time, but Mark Anthony and Octavius, members of the second Roman Triumvirate.
This is also the scene where Brutus and Cassius say their goodbyes in case they never meet again.